[sdiy] Analog vs. Digital grounds and Doepfer power bus.

[Michael Baxter]

Hi Bret,

I'll try to explain analog vs. digital grounds, but not the Doepfer power 
bus, which I have no experience with.

The main issue with ground is that for every power supply voltage that 
sources current I to circuitry, there is a return current I' that goes 
back to the power supply, from somewhere, typically a ground bus.

This is true for both digital and analog circuits. In the case of digital 
circuits, very often the current being drawn and returned is impulsive -- 
it's needed very quickly (in say 1-100 nS, depending on the circuitry) in 
order to raise or lower a voltage step somewhere in the system. In digital 
circuits, there's a lot of this going on concurrently, making it an 
impulsive or "spikey" electronic noise environment.

If there are not good return paths to ground for these currents, they will 
"leak" or "contaminate" other circuitry, particularly for example 
sensitive analog circuits that utilize current on a continuous or "AC" 
basis, but perhaps at very low level (.1 - 100 uA or so).

The contamination occurs because current always follows the path of least 
resistance. In a well-designed system, this is a the ground return path. 

A good strategy for minimizing impulsive noise interactions between 
digital and analog circuits is in fact to use separate ground returns.

These will meet at the point of least resistance (or impedance, in terms 
of AC, or high frequency use of current): the power supply ground, quite 
close to the power supply.

A wholy plausible scenario in a synthesizer is there are +12V or +15V and 
-12V and -15V supplies for linear or analog circuitry, and a +5V supply 
for digital circuitry -- so, three grounds, one for each supply voltage.

Hope this is a helpful explanation!

Best,
Michael

On Tue, 25 Nov 2003, Bret Truchan wrote:

> 
> Hello!
> 
> I love this forum!  I'm a little confused.  I've been told that it's a good 
> idea to keep ICs grounds and analog grounds separate.  When I look at my 
> Doepfer power bus, it has three grounds.  Do the Doepfer modules utilize the 
> different grounds to keep their analog and digital ground separate?  If so, 
> how do you know which one to use for what?
> 
> Or, are the three grounds on the power bus tied together?
> 
> Thanks!
> - Bret
> 
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